Chemical strengthening, also called ion-exchange (IOX), of glass refers to the exchange of smaller cation (e.g., monovalent alkali metal cations such as Li+ and Na+) in the glass for larger monovalent cations (e.g., K+) in an external medium, such as a molten salt bath at temperatures below the strain point of the glass. The ion exchange process is used to impart a compressive stress profile extending from the surface of the glass to a particular depth following a complementary error function. High compressive stress gives high strength in bending as long as the flaw is encompassed in the depth of the compressive layer (depth of layer, or “DOL”).
The stored compressive stress from both surfaces of the glass is balanced by stored tension, the allowable limit of which is set by the frangibility limit for a given glass thickness. The limits of compressive stress and depth of layer are determined by the various allowable combinations that follow the complementary error function and remain below the frangibility limit. The stored compressive stress is represented by the area under the complementary error function from the surface to the depth of layer.